Antarctic Ice Reveals an 80,000-Year Clue to the Solar System's Path Through Stardust

Antarctic ice holds a radioactive clue to the Solar System's journey through interstellar space. Researchers studying Antarctic snow and ice have found traces of iron-60, a radioactive isotope made only in exploding stars. The find, published in Physical Review Letters, helps trace how the Solar System has moved through nearby clouds of gas, plasma and stardust over the past 80,000 years.

Massive stars forge elements such as iron in their cores. When they explode as supernovae, some material — including the rare isotope iron-60 — is ejected into space as interstellar dust, and tiny grains occasionally reach Earth. Antarctica is a valuable place to look for it: its snow builds up slowly and undisturbed, forming a layered record stretching back tens of thousands of years.

The team had earlier found iron-60 unexpectedly in 500 kilograms of recent Antarctic snow, even though no supernova has occurred near Earth recently. One idea was that the Solar System is picking up stardust while passing through one of about 15 interstellar clouds nearby. To test this, researchers analysed a 300-kilogram section of Antarctic ice dating from 40,000 to 80,000 years ago, isolating iron-60 and counting individual atoms with accelerator mass spectrometry at the Australian National University.

They found less iron-60 than expected from earlier measurements of surface snow and ocean sediments — not zero, but noticeably lower — suggesting less interstellar dust reached Earth during that period. A separate study last year concluded the Solar System has been passing through the nearby Local Interstellar Cloud since sometime between 40,000 and 124,000 years ago, matching the timing of the drop the team observed.

The match is not perfect: if the local clouds had formed directly from a nearby exploding star, researchers would expect far more iron-60 than they actually measured. The team says analysing even older ice could help unravel how these interstellar clouds formed.